Testing private members is not always about verifying the state by checking if it equals some expected values. In order to accommodate other, more intricate test scenarios, I sometimes use the following approach (simplified here to convey the main idea):
// Public header
struct IFoo
{
public:
virtual ~IFoo() { }
virtual void DoSomething() = 0;
};
std::shared_ptr<IFoo> CreateFoo();
// Private test header
struct IFooInternal : public IFoo
{
public:
virtual ~IFooInternal() { }
virtual void DoSomethingPrivate() = 0;
};
// Implementation header
class Foo : public IFooInternal
{
public:
virtual DoSomething();
virtual void DoSomethingPrivate();
};
// Test code
std::shared_ptr<IFooInternal> p =
std::dynamic_pointer_cast<IFooInternal>(CreateFoo());
p->DoSomethingPrivate();
This approach has the distinct advantage of promoting good design and not being messy with friend declarations. Of course, you don't have to go through the trouble most of the time because being able to test private members is a pretty nonstandard requirement to begin with.
